diff --git a/Data/String/cork.cpp b/Data/String/cork.cpp
index 67c0689..75f5f6d 100644
--- a/Data/String/cork.cpp
+++ b/Data/String/cork.cpp
@@ -29,12 +29,11 @@ contains_unicode_char (string s) {
 string
 tm_encode (string s) {
   // verbatim to TeXmacs encoding
-  int    i;
   string r;
-  for (i= 0; i < N (s); i++) {
-    if (s[i] == '<') r << "<less>";
-    else if (s[i] == '>') r << "<gtr>";
-    else r << s[i];
+  for (const auto ch : s) {
+    if (ch == '<') r << "<less>";
+    else if (ch == '>') r << "<gtr>";
+    else r << ch;
   }
   return r;
 }
diff --git a/tests/Data/String/cork_test.cpp b/tests/Data/String/cork_test.cpp
new file mode 100644
index 0000000..1c47572
--- /dev/null
+++ b/tests/Data/String/cork_test.cpp
@@ -0,0 +1,22 @@
+
+/******************************************************************************
+ * MODULE     : cork_test.cpp
+ * DESCRIPTION: tests for the Cork encoding
+ * COPYRIGHT  : (C) 2024    Darcy Shen
+ *******************************************************************************
+ * This software falls under the GNU general public license version 3 or later.
+ * It comes WITHOUT ANY WARRANTY WHATSOEVER. For details, see the file LICENSE
+ * in the root directory or <http://www.gnu.org/licenses/gpl-3.0.html>.
+ ******************************************************************************/
+
+#include "cork.hpp"
+#include "modification.hpp"
+#include "moe_doctests.hpp"
+#include "tree.hpp"
+
+TEST_CASE ("tm_encode") {
+  string_eq (tm_encode ("<>"), "<less><gtr>");
+  string_eq (tm_encode ("<#ABCD>"), "<less>#ABCD<gtr>");
+  string_eq (tm_encode ("abc"), "abc");
+  string_eq (tm_encode (""), "");
+}